This invention relates to integrated circuit (“IC”) devices. More particularly, this invention relates to output buffer circuitry in IC devices.
IC devices, such as programmable logic devices (“PLDs”), application-specific integrated circuits (“ASICs”), or devices having characteristics of both PLDs and ASICs, often need to transmit relatively high-frequency data signals robustly over relatively long distances. Various standards have been developed to facilitate such transmission, such as low-voltage differential signaling (“LVDS”).
LVDS utilizes the voltage difference between a pair of signals to represent a logical value. For instance, a logical 1 can be transmitted by driving the voltage of a first signal to a relatively high value, while the voltage of a second signal is driven to a relatively low value. Similarly, a logical 0 can be transmitted by driving the voltage of the first signal to a relatively low value, while the voltage of the second signal is driven to a relatively high value.
Many relatively high-end IC devices utilize dedicated LVDS output buffer circuitry to perform LVDS transmission. This dedicated circuitry is often capable of performing signal pre-emphasis by increasing the buffer drive strength. Pre-emphasis can be beneficial in many ways. For instance, a higher drive strength can generate faster signal transitions, resulting in sharper signal waveforms (e.g., a wider “eye diagram”). Effective pre-emphasis can also reduce jitter and allow the use of less expensive interconnect. Overall, this improved data integrity can permit the use of relatively high data rates over relatively long distances. The use of a high transmission frequency also reduces the chance of inter-symbol interference. In many IC systems, the pre-emphasis of an output driver is often programmable, allowing the user to specify a desired drive strength.
Unfortunately, dedicated LVDS output buffers with programmable pre-emphasis often require relatively large transistors to generate the necessary current levels. As a result, such drivers often consume relatively large amounts of area and are relatively expensive to manufacture, making them unsuitable for some applications.
One possible solution is to use programmable non-dedicated output buffer circuitry that can support several output current strengths. A pair of such output buffers can be used for LVDS transmission by selecting appropriate current strengths and coupling their outputs to an external resistor network. Unfortunately, such a solution often does not support pre-emphasis of the transmitted signal. As a result, it is difficult to transmit relatively high-frequency signals across a relatively long distance.
In view of the foregoing, it would be desirable to provide methods and apparatus that perform LVDS pre-emphasis using output buffer circuitry that is not dedicated to LVDS transmission. In addition, it would be desirable to make the output current drive strength of such buffer circuitry programmable. It would also be desirable to provide such benefits without significantly increasing the power consumption or area of the output buffer circuitry.